B cells from Group 2 X-SCID/JAK3-deficient patients fail to proliferate in response to CD40L/IL-21 stimulation. (A-C) PBMCs were labeled with CFSE and were either unstimulated or stimulated with CD40L/IL-21 or CpG. After 4 days, proliferation of B cells was assessed by incubating the cultured cells with anti-CD19 mAb and determining dilution of CFSE by CD19⁺ cells by flow cytometric analysis. (A-B) Percentage of CFSE^lo cells in cultures of PBMCs from the indicated donor groups after stimulation with CD40L/IL-21 (A) or CpG (B). (C) Representative FACS plots of CFSE dilution in PBMCs isolated from the indicated donor groups after stimulation in the absence or presence of CD40L/IL-21. Statistical analysis was performed using 1-way ANOVA and Bonferroni posttest analysis (**P < .01; ***P < .001).